Method and apparatus for sensing downhole well conditions in a wellbore

ABSTRACT

A method and apparatus for sensing downhole well conditions in a wellbore having a drill string suspended therein and pump means and conduit means for circulating drilling fluid down the well. The apparatus includes a pressure transmitter for sensing the circulating pressure of input drilling fluid being circulated down the well and generating a first signal representative thereof. It could also include means in the form of a differential pressure transmitter for sensing a pressure drop of the drilling fluid along a portion of the conduit means and generating a second signal representative thereof. Readout means in the form of dual concentric pressure gauges are provided for reading out a change between the signals as an indication of a change in downhole well conditions, such as loss of drilling fluids or incursion of formation fluids into the well.

71 Xi"; 3e595e75 -n-uvu uvuwvu l. tbllt [H] [72] Inventor Ethell J. Dower 3,504,537 4/1970 Cline 73/407 X Houston Primary Examiner-Jerry W. Myracle g 1969 Att0rneys-Paul E. Harris and Lee R, Larkin 1 e ov. [45) Patented July 27, 1971 173] Assignee Warren Automatic Tool Company Houston, Tex.

ABSTRACT: A method and apparatus for sensin downhole g [54] METHOD AND APPARATUS FOR SENSING well conditions in a wellbore having a drill string suspended DOWNHOLE WELL CONDITIONS IN A therein and pump means and conduit means for circulating drilling fluid down the well. The apparatus includes a pressure :nwin s transmitter for sensing the circulating pressure of input 8 lg drilling fluid being circulated down the well and generating a U.S. first ignal representative thereof It could also include means 175/48 in the form of a differential pressure transmitter for sensing a [5 l Int. Cl E2") pressure dro of the along a portion of the con- [50] Field of Search 73/ 155, d n means d generating a second signal representative 21 407, 152; 175/48 thereof. Readout means in the form of dual concentric pressure gauges are provided for reading out a change between the [56] Re'erences Cned signals as an indication of a change in downhole well condi- UNITED STATES PATENTS tions, such as loss of drilling fluids or incursion of formation 2,776.817 1/1957 Gregory et al. 175/48 fluids into the well.

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PRESS. TRA NS- #11 ETHELL J. DOWER INVENTOR ATTORNEY METHOD AND APPARATUS FOR SENSING DOWNIIOLE WELL CONDITIONS IN A WELLBORE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method and apparatus for sensing downhole well conditions in a wellbore having a drill string suspended therein and pump means and conduit means for circulating drilling fluid down the well. More particularly, the invention relates to a method and apparatus for sensing a change in downhole well conditions, such as a loss of drilling fluid to the formation or the incursion of formation fluids into the well, for example.

2. Description of the Prior Art During the drilling of wellbores, as for example oil wells, it is common practice to drill the well with a rotary bit suspended on the end of a drill string in the wellbore. A drilling fluid, such as mud, is circulated down the drill pipe to the bit and up the annulus or vice versa. The drilling mud serves the purpose of flushing cuttings from the wellbore, cooling the bit, and providing a hydrostatic pressure to match formation pressures to thereby prevent a possible blowout condition from developing. If the condition should occur that the hydrostatic pressure of the mud plus any back pressure which may be applied to the wellbore, are not sufficient to match formation pressure, then there may be an incursion of formation fluids into the wellbore and create what is commonly referred to as a kick. Unless properly controlled and circulated out, such a kick can result in a blowout and subsequent loss of the hole and destruction of property and lives. In the event that the hydrostatic pressure of the drilling fluid in the wellbore is in excess of formation pressure along the uncased portions of the wellbore, then there is a possibility of loss of circulation wherein the drilling fluid flows into the formation and there is no return drilling fluid to the surface.

In order to overcome the aforesaid problems, it has been common practice in the drilling industry to pump drilling mud of the desired weight, density and characteristics down the drill pipe and sense the circulating drill pipe pressure. During such pumping, the circulating drill pipe pressure is particularly useful information when circulating out a kick," so that the proper amount of back pressure can be applied to the well to prevent further incursions of drilling fluid.

The drill pipe pressure, or the pressure required to circulate the drilling fluid through the well, can be a very important measurement, not only in killing a well kick," but also doing drilling operations. Since the circulating drill pipe pressure represents the sum total of all pressure drops through the well circulation system, it is a good indicator of changes and conditions in the wellbore.

One of the obstacles to the use of circulating drill pipe pressure as an early indicator of downhole trouble is the fact that it is a resultant of many factors and most of them are subject to change. For example, the circulating drill pipe pressure is affected by a number of factors, including circulation rate or the rate at which the drilling mud is pumped down the wellbore. Increased circulation rate increases drill pipe pressure as a function of the square of the velocity resulting from the flow rate. Moreover, increases in density and viscosity of the drillingmud cause increased drill pipe pressure. Drill pipe pressure also increases with the depth because of the increased, friction inside the drill pipe, as well as in the annulus of the wellbore. Drill pipe pressure also increases as the annular cross-sectional area between the wellbore wall and the drill pipe decreases. Drill pipe pressure also increases with smaller bit nozzles.

Drill pipe pressure increases as surface back pressure is applied to the return fluid, unless the casing pressure increases are offset by expanding gas displacing mud from the annulus, as for example, that which occurs during the circulation of a kick and wherein the operator maintains the drill pipe pressure constant.

Drill pipe pressure may also increase if a high pressure, high production formation produces formationfluid into the annulus, because the annulus flow increases and this increases annulus friction. Drill pipe pressure may decrease after sufficient incursion has reduced annulus hydrostatic pressure. Normally drill pipe pressure will decrease when there is a loss of circulation, as for example the loss of drilling fluid to the formation, as discussed above, because this reduces annulus friction.

The largest part of the total drill pipe circulating pressure comes from pressure drop across the bit nozzles. This may be on the order of percent of the total pressure drop. Annular friction may be only about 10 percent or less of the total. Aside from detection of drill pipe washouts (a hole in the drill pipe, for example) and the loss of nozzles from the bit, both of which alter the large 80 percent figure noted above, the hole conditions changes will alter only the small percent factor of annular friction. This small percentage change resulting from hole conditions is difiicult to observe and correlate because of variations in circulation rate. Small variations in circulation rate commonly occur as a result of mud pump speed variations and variation in mud pump efflciency. These changes in circulation cause changes in the total drill pipe circulating pressure which are large compared to the annular friction portion and thus mask the small but important changes which occur in this portion of the hole.

During the killing of a kick it is common practice to continuously regulate surface back pressure in a manner which produces a constant drill pipe circulating pressure, as this assures a correct applied bottomhole pressure, provided, of course, that circulation rate is held constant. Here the adjustment of the constant pump speed still leaves the variable pump efficiency as a factor which can cause an error.

SUMMARY OF THE INVENTION It is therefore an object of this invention to provide an improved method and apparatus for sensing changes in downhole well conditions, which method and apparatus are not subject to the foregoing limitations.

Briefly stated, this invention includes a method for sensing downhole well conditions in a wellbore having a drill string suspended therein and pump means and conduit means for circulating drilling fluid down the well. It includes in combination the steps of sensing the circulating pressure of input drilling fluid, commonly referred to as circulating drill pipe pressure, being circulated down the well and generating a first signal representative thereof. It also includes sensing a flow characteristic of the drilling fluid in the conduit means, such as the pressure drop of the drilling fluid along a portion of the conduit means, and generating a second signal representative thereof. It also includes monitoring these two signals to detect a change therebetween as an indication of change in downhole well conditions. The sensing of the pressure drop may be accomplished by sensing circulating drilling fluid pressure at two spaced apart points along the conduit means, comparing the pressures at said points, and generating a signal representative of the difference therebetween as the second signal. Further embodiments may include the step of multiplying the second signal by a factor such that the first and second signal are of substantially the same magnitude during normal operations, whereby the difference between the first signal and the multiplied second signal indicates an abnormal condition in the well.

The apparatus of this invention is for sensing downhole well conditions in a wellbore having a drill string suspended therein and pump means and conduit means for circulating drilling fluid down the well. The combination of the invention includes means for sensing the circulating pressure of input drilling fluid being circulated down the well and generating a first signal representative thereof. Means are also provided for sensing a flow characteristic of the input drilling fluid in the conduit means, which characteristic may be of a pressure drop along the conduit means, and generating a second signal representative thereof. Readout means are included which are connected with the sensing means for reading out a change between the signals an indication ofa change in downhole well conditions.

Certain embodiments may include means for sensing pressure drops along two points in the conduit means and means for comparing the pressures at the two points and generating a signal representative of the differences therebetween, as the second signal. Means may also be provided for multiplying the second signal to increase the value thereof relative to the first signal. Other embodiments may include means for flowing at least a portion of the drilling fluid therethrough. The readout means may include a pair of concentric gauges, one of which is responsive to the first signal and one of which is responsive to the second signal.

BRIEF DESCRIPTION OF THE DRAWINGS Reference to the drawings will further explain the invention, wherein like numerals refer to like parts, and in which:

FIG. 1 is a schematic diagram of one embodiment of the invention.

FIG. 2 is a schematic diagram of another embodiment of the invention. DESCRIPTION OF THE PREFERRED EMBODI- MENT Referring now to FIG. 1, the means for circulating drilling fluid down a wellbore includes a mud pump 11 connected to flow mud through conduit means in the form of mud line 12, which is connected to the conventional stand pipe of the drilling rig and which is arranged to flow the drilling mud down the bore of the drill pipe to the drill bit and up the annulus. Means are provided for sensing the circulating pressure of input drilling fluid being circulated to the well and generating a first signal representative thereof which is in the form of a pressure transducer 13 which is connected to line 12 and is supplied with a regulated air supply on line 14. Pressure transducer 13 is of conventional design and provides an analog pneumatic output signal on the order of 315 p.s.i. on line 15. Line 15 is connected to operate one indicator needle of dual concentric indicator gauge 16 which is appropriately calibrated to cover the expected range of drill pipe pressures to be encountered.

Means are also provided for sensing a flow characteristic, as for example the pressure drop, of the drilling fluid along a portion of the conduit means and generating a second signal representative thereof. This includes diaphragm boxes 19 and 20 which are spaced along mud line 112 and are respectively connected by hydraulic lines Zll and 22 to differential pressure transducer 23 which is supplied with a regulator air supply over line 24. Differential pressure transducer 23 is of conventional design and is arranged to have a pneumatic output signal on the order of 3-15 p.s.i. on line 26. Differential pressure transducer 23 is arranged to receive the high pressure signal on line 21 and the low pressure signals from line 22. in other words, the pressure will be somewhat lower at diaphragm box 20 than at diaphragm box 19 because of friction losses in the mud conduit. It is to be understood that diaphragm boxes 19 and 20 may be spaced several feet apart, on the order of 20 to 60 feet, for example. Alternatively, flow restriction means in the form of flow restrictor 30 are connected by lines 31 and 32 to mud line 12, as shown. In this instance, meter bypass valve 33 may be interposed in mud line 12 and arranged such that at least a portion of the input drilling fluid is flowed through flow restrictor 30 to thereby induce a small amount of friction loss in the conduit means for circulating the drilling fluid pressure to the well.

The output on line 26 may sometimes be referred to as the second signal and may be multiplied to be of the same general magnitude as the pressure signal on line 15. This multiplication is accomplished by a multiplier analog computer 36 which is connected to line 26 and is arranged to receive a regulated air supply on line 37. It is also arranged to receive a 3-5 p.s.i. pneumatic pressure signal on line 38, which is connected to pressure regulator 39 also connected to a regulated air supply line All). Conveniently, line 38 may be connected to a regulator gauge 411 so that regulator 39 may be manipulated to provide the correct multiplier to multiplier analog computer 36. The output of multiplier analog computer 36 is to line 42 carrying a pneumatic analog signal on the order of 3 15 psi. which pressure is applied to the other needle of dual concentric indicator gauge 16, which needle is appropriately calibrated to cover the expected pressure variations which are to be encountered between diaphragm boxes 19 and 20.

In operation, the apparatus would be set up as shown in FIG. I and as described above. The needles of dual concentric indicator gauge to would be calibrated to overlay each other initially during normal drilling operations. During subsequent drilling operations, if there should be a change in downhole well conditions, such as a loss of circulation or the incursion of drilling fluid into the wellbore, for example, such changes would be reflected by the divergence of the needles of dual concentric indicator gauge 16, which needles would assume a spaced relationship to each other.

By this apparatus, flow changes, such as those sensed by diaphragm boxes 19 and 20, are mathematically related to circulating pressure during normal drilling operations and before unusual or abnormal conditions are encountered. This apparatus greatly improves the usefulness of drill pipe circulating pressure as an indicator of downhole well conditions. The apparatus of this invention therefore compares measured drill pipe pressure with a pneumatically computed drill pipe pressure. The pressure drop which is sensed between diaphragm boxes l9 and 20 is analogous to the drill pipe, bit nozzle, and annulus of the wellbore. Differential pressure detected between diaphragm boxes l9 and 20 is generally proportional to the velocity squared, density and viscosity of the mud, the same as the drill pipe circulating pressure is related to these items. In the event that flow restrictor 30 is used it is to be understood that the resistance offered thereby will be made low, as for example on the order of only a few percentage points of drill pipe pressure. It is kept low so that the increased load on the mud pump 111 is not significant. The change in pressure between diaphragm boxes I19 and 20 is converted to a 3-5 p.s.i. pressure signal by difierential pressure transducer 23, which is of conventional design of the type commonly used to meter flow. Because of the configuration of the mud line 12 and/or flow restrictor 30, the output from differential pressure transducer 23 would have a fixed relation to flow, density, viscosity, etc. The restriction to flow in the wellbore is subject to change with depth, bit nozzle size, hole size, etc., as discussed above. Rather than alter the calibration of the flow meter to match these changing well conditions, it may be preferable to put the signal through a computing relay such as multiplier analog computer 316 with an adjustable multiplier factor so that the computer output can be fit to be exactly equivalent to drill pipe pressure for the existing well conditrons.

With multiplier analog computer 336 so adjusted, any subsequent change in well conditions will be reflected as a change in drill pipe pressure transmitted on line 115, but not in the computed drill pipe pressure transmitted via lines 26 and 42. This difference is measured and readout by dual concentric indicator gauge to or alternatively by using another differential pressure transducer, the output of which is proportional to the difference between the first and second signals discussed above, or by appropriate record means.

A differential transmitter is optional and may take the form of differential pressure transducer 415, which is connected via line as to line 42 and by line 47 to line 15, and with the out puts thereon connected to differential pressure gauge 48 by line 49. The output from transducer 45 may be a pneumatic signal on the range of 3-15 p.s.i. which is proportional to the difference between the two signals applied thereto.

Hence, any appreciable change in flow rate, density or viscosity will alter the signals on lines 315 and 42 by like amounts and no difference therebetween will occur or be detected. Hence, an observer may monitor either dual concentric indicator gauge 16, or differential pressure gauge 48 or other readout means in the form of recorders or the like and can determine the difference in measured and computed drill pipe pressures, and any change therebetween is attributed to some downhole change in the circulating system and serves as an immediate warning of trouble.

It is to be understood that if differential pressure transducer 23 is sufficiently convenient and has sufficient range, it can be used in lieu of multiplying analog computer 36 to set the computed drill pipe pressure equal to the measured drill pipe pressure for existing well conditions.

A typical example of suitable differential pressure transdu cers 23 and 45 is one sold by Foxboro Company of Foxboro, Mass. and bearing Model No. 1311.

A typical example of a suitable multiplier analog computer 36 is one sold by Foxboro Company of Foxboro, Mass. and bearing Model No. 556.

A typical example of pressure transducer 13 is one sold by Foxboro Company of Foxboro, Mass, and bearing Model No. l lGH.

Referring not to FIG. 2, an alternate and perhaps even more preferred embodiment will be described. In this instance, a mud pump 50 is shown connected to a conventional mud line 51 leading to the stand pipe of the drilling rig. Two diaphragm pressure sensor boxes 52 and 53 are connected to mud line 51 at substantially spaced apart positions and arranged such as to be responsive to the pressure of mud in line 51 at the points where they are positioned. The spacing between boxes 52 and 53 may be on the order of to 60 feet and preferably at least 40 to 60 feet. Box 52 is connected by hydraulic line 55 to pressure transducer 56 which is connected to a source of air supply over line 57 and is arranged to have a pneumatic output in the range of 3-15 p.s.i. on line 58, which in turn is connected to drill pipe pressure gauge 59. it is to be understood that gauge 59 is one pan of a dual concentric indicator gauge, such as indicator gauge 16 shown in the previous embodiment.

Diaphragm pressure sensor box 52 is also connected by hydraulic line 60, which in turn is connected as one of the inputs to differential pressure transducer 61 and having an air supply input on line 62. Differential pressure transducer 61 is connected by hydraulic line 63 to diaphragm pressure sensor box 53 and is arranged to have a pneumatic output signal on line 64 spanning the range of about 3-15 p.s.i., which signal is representative of the difference between the pressures sensed by boxes 52 and 53.

It is to be understood that pressure transducer 56 is similar to pressure transducer 13, described in the previous embodiment, and differential pressure transducer 61 is similar to differential pressure transducer 23, previously described.

Line 66 connects via line 64a to adjustable proportioning valve 65, which in turn is connected by line 66b to bleed valve 67 which is provided with an air supply over line 68 which also connects to an optional booster valve 69, which is applied to well circulating pressure gauge 70 having a needle which is deflected in response to pressure applied on line 71 from booster valve 69. It is to be understood that gauge 70 is calibrated to cover the anticipated range of computed well circulating pressure and is concentric with respect to drill pipe pressure gauge 59 and is similar in construction to dual concentric indicator gauge 16 described with respect to the previous embodiment.

Theapparatus also includes a line 73 connected between line 72 and line 71, the purpose of which is to optionally bypass booster valve 69 which is a two-to-one multiplier. A fixed restrictor 74 is provided in line 64 and valve 65 is a variable restrictor in line 64a. Valve 67 is a regulator connected to line 64b. Regulator 67 is set to have an output of 3 p.s.i. which is equal to the zero 3 p.s.i. signal in line 64 from differential pressure transducer 61. Regulator valve 67 will bleed off any pressure in excess of 3 p.s.i. in line 64b. The adjustment of restrictor valve 65, in conjunction with fixed restrictor 74,

modifies the pressure of line 64 as it exists in line 64a and line 72. if restrictor valve 65 is closed, pressure in line 64a and line 72, is equal to that in line 64. This could be called the 100 percent setting. lf restrictor valve 65 is opened, it bleeds off pressure in line 64a down to near 3 p.s.i. (zero). This could be called the 10 percent setting. At intermediate settings it can cause pressure in line 64a to be any fraction of that in line 64, as for example 50 percent. With booster relay 69 in operation, the 100 percent setting of restrictor valve 65 causes a 100 percent signal in line 64a and line 72 into the two-to-one booster relay 69 and the result is a 200 percent signal pressure in output line '71 to indicator gauge 70. With a 50 percent setting on restrictor valve 65, the 50 percent signal pressure in line 72 results in a 100 percent output signal in line 71 to indicator gauge 70. Restrictor valve 65, in conjunction with booster relay 69, can modify signal pressure existing in line 64 by 20 percent to 200 percent in line 71 and indicator 70. if booster relay 69 is bypassed by line 73, then the pressure in line 72 is conducted directly to indicator gauge and the resultant modification range is l0 percent to 100 percent.

It is to be understood that the total assembly found within the dotted square is a Moore Products M/P ratio control apparatus sold by Moore Products Company of Spring House, Pa., under the Model No. 543.

In operation, valve 65 is adjustably set for well conditions so that the pressure signal applied to well circulating pressure gauge 70 equals the pressure applied to drill pipe pressure gauge 59 during normal drilling operation.

Upon encountering abnormal well conditions, as for example an incursion of drilling fluid into the wellbore or loss of circulation, a difference will be noted on gauges 59 and 70. Hence, by monitoring these gauges, any variation therebetween would indicate a potentially hazardous condition, the same as would be indicated by dual concentric indicator gauge 16 of the previous embodiment.

Further modifications and alternate embodiments will be apparent to those skilled in the art in view of this description. Accordingly, the foregoing description is to be construed as illustrative only for teaching those skilled in the art how to build and perform the invention.

What 1 claim is:

1. In a method for sensing down hole well conditions in a well bore having a drill string suspended therein and pump means and conduit means for circulating drilling fluid down said well, the combination of steps comprising:

sensing the circulating pressure of input drilling fluid being circulated down said well and generating a first signal representative thereof;

sensing drilling fluid pressure at two points along said conduit means with one of said points being upstream with respect to the other of said points;

comparing the pressures at said points and generating a second signal representative of the difference therebetween;

multiplying said second signal by a factor such that said first and second signals are of substantially the same magnitude during normal operations; and

monitoring said signals to detect a change therebetween as an indication of a change in down hole well condition.

2. in a method for sensing down hole well conditions in a well bore having a drill string suspended therein and pump means and conduit means for circulating drilling fluid down said well, the combination of steps comprising:

sensing the circulating pressure of input drilling fluid being circulated down said well and generating a first signal representative thereof;

passing at least a portion of said drilling fluid through a flow restriction in said conduit means;

sensing drilling fluid pressure at two points along said conduit means with one of said points being upstream and the other of said points being downstream from said flow restriction;

comparing the pressures at said points and generating a second signal representative of the difference therebetween;

and, monitoring said signals to detect a change therebetween as an indication of a change in down hole well conditions.

3. The invention as claimed in claim 2 wherein:

said flow restriction is varied to thereby cause said first and second signals to be substantially of the same magnitude during normal operations, whereby a difference between said first and second signals indicates an abnormal condillOn.

4. In a method for sensing down hole well conditions in a well bore having a drill string suspended therein and pump means and conduit means for circulating drilling fluid down said well, the combination of steps comprising:

sensing the circulating pressure of input drilling fluid being circulated down said well and generating a first signal representative thereof;

sensing the pressure drop of said drilling fluid along a portion of said conduit means and generating a second signal representative thereof;

and, comparing said first and second signals and generating a third signal representative of any change therebetween.

5. In apparatus for sensing down hole well conditions in a well bore having a drill string suspended therein and pump means and conduit means for circulating drilling fluid down said well, the combination comprising:

means for sensing the circulating pressure of input drilling fluid being circulated down said well and generating a first signal representative thereof;

means for sensing drilling fluid pressure at two points along said conduit means, with one of said points being upstream with respect to the other of said points;

means for comparing the pressures at said points and' generating a second signal representative of the difference therebetween; I

means for multiplying said second signal to increase the value thereof relative to said first signal;

- and, readout means for reading out a change between said first signal and said multiplied second signal as an indication ofa change in down hole well conditions.

6. ln apparatus for sensing down hole well conditions in a well bore having a drill string suspended therein and pump means and conduit means for circulating drilling fluid down said well, the combination comprising:

means for sensing the circulating pressure of input drilling fluid being circulated down said well and generating a first signal representative thereof;

means for sensing drilling fluid pressure at two points along said conduit means, with one of said points being upstream with respect to the other of said points;

means for comparing the pressures at said points and generating a second signal representative of the difference therebetween;

means for comparing said first and second signals and generating a third signal representative of any change therebetween:

and, readout means connected to receive said third signal for reading out a change in said third signalas an indication ofa change in down hole well conditions.

U. S. PATENT OPIICE UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 595, 75 Dated 7/ 7/71 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Ethell J. Dower Column 3, line 3, after "signals" insert Column 3, line ll, after "include" insert -a flow restriction-- Column 5, line 22, change "not" to now- Signed and sealed this 22nd day of February 1972.

(SEAL) Attest:

EDWARD M.FLETCHE.R,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

1. In a method for sensing down hole well conditions in a well bore having a drill string suspended therein and pump means and conduit means for circulating drilling fluid down said well, the combination of steps comprising: sensing the circulating pressure of input drilling fluid being circulated down said well and generating a first signal representative thereof; sensing drilling fluid pressure at two points along said conduit means with one of said points being upstream with respect to the other of said points; comparing the pressures at said points and generating a second signal representative of the difference therebetween; multiplying said second signal by a factor such that said first and second signals are of substantially the same magnitude during normal operations; and monitoring said signals to detect a change therebetween as an indication of a change in down hole well condition.
 2. In a method for sensing down hole well conditions in a well bore having a drill string suspended therein and pump means and conduit means for circulating drilling fluid down said well, the combination of steps comprising: sensing the circulating pressure of input drilling fluid being circulated down said well and generating a first signal representative thereof; passing at least a portion of said drilling fluid through a flow restriction in said conduit means; sensing drilling fluid pressure at two points along said conduit means with one of said points being upstream and the other of said points being downstream from said flow restriction; comparing the pressures at said points and generating a second signal representative of the difference therebetween; and, monitoring said signals to detect a change therebetween as an indication of a change in down hole well conditions.
 3. The invention as claimed in claim 2 wherein: said flow restriction is varied to thereby cause said first and second signals to be substantially of the same magnitude during normal operations, whereby a difference between said first and second signals indicates an abnormal condition.
 4. In a method for sensing down hole well conditions in a well bore having a drill string suspended therein and pump means and conduit means for circulating drilling fluid down said well, the combination of steps comprising: sensing the circulating pressure of input drilling fluid being circulated down said well and generating a first signal representative thereof; sensing the pressure drop of said drilling fluid along a portion of said conduit means and generating a second signal representative thereof; and, comparing said first and second signals and generating a third signal representative of any change therebetween.
 5. In apparatus for sensing down hole well conditions in a well bore having a drill string suspended therein and pump means and conduit means for circulating drilling fluid down said well, the combination comprising: MEANS for sensing the circulating pressure of input drilling fluid being circulated down said well and generating a first signal representative thereof; means for sensing drilling fluid pressure at two points along said conduit means, with one of said points being upstream with respect to the other of said points; means for comparing the pressures at said points and generating a second signal representative of the difference therebetween; means for multiplying said second signal to increase the value thereof relative to said first signal; and, readout means for reading out a change between said first signal and said multiplied second signal as an indication of a change in down hole well conditions.
 6. In apparatus for sensing down hole well conditions in a well bore having a drill string suspended therein and pump means and conduit means for circulating drilling fluid down said well, the combination comprising: means for sensing the circulating pressure of input drilling fluid being circulated down said well and generating a first signal representative thereof; means for sensing drilling fluid pressure at two points along said conduit means, with one of said points being upstream with respect to the other of said points; means for comparing the pressures at said points and generating a second signal representative of the difference therebetween; means for comparing said first and second signals and generating a third signal representative of any change therebetween: and, readout means connected to receive said third signal for reading out a change in said third signal as an indication of a change in down hole well conditions. 